#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i, n) for(int i=0;i<(n);++i)
#define rep1(i, n) for(int i=1;i<=(n);i++)
#define ll long long
using mint = modint998244353;
using P = pair<ll,ll>;
using lb = long double;
using T = tuple<ll, ll, ll>;
#ifdef LOCAL
#  include <debug_print.hpp>
#  define dbg(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define dbg(...) (static_cast<void>(0))
#endif

int main()
{
    ll m, n;
    cin >> m >> n;
    vector<ll> x(n);
    rep(i,n) cin >> x[i];
    mint ans = 0;
    ll now = 0;
    x.push_back(m+1);
    rep(i,n+1) {
        mint val = x[i] - now - 1;
        now = x[i];
        ans += val*(val+1)*(2*val+1)/6;
    }
    cout << ans.val() << endl;
    return 0;
}